1. Field of the Invention
This invention relates to the preparation of modified aluminas and their use in cracking catalysts. More particularly, the invention relates to the preparation of modified aluminas using low-molecular weight organic acids and their use in fluidizable cracking catalysts for cracking heavy hydrocarbon feedstocks, especially those containing metals.
2. Summary of the Prior Art
Heavy hydrocarbon feedstocks generally are contaminated by various materials, including metals. More particularly at issue here are nickel and vanadium contaminations, since during the catalytic cracking of such feedstocks they will be captured by the cracking catalyst and poison it. Such poisoning causes reduced catalyst activity and a decrease of the selectivity to valuable cracking products such as gasoline, together with the production of additional quantities of objectionable products such as hydrogen, gas, and coke. Research has shown that the presence of nickel leads to increased hydrogen and coke production, as indeed does vanadium to some extent, although the principal effect of vanadium is that it attacks the catalyst's zeolite structure, as a result of which there is deterioration of the catalyst activity. The prior art has provides a wide range of proposals for solving this problem of catalytically cracking heavy, metals-containing feedstocks.
Thus, U.S. Pat. No. 4,283,309 proposes a cracking catalyst especially suited for use in cracking heavy, metals-containing feedstocks which is composed of a crystalline aluminosilicate, an inorganic oxide gel, and a porous inorganic oxide. The surface area of this porous inorganic oxide is greater than 200 m g and at least 0.2 ml/g of the pore volume should be in pores ranging in diameter from 90 to 200 .ANG.; these parameter values were determined on the material after its calcination at 538.degree. C. for 6 hours and independent of the other catalyst components. Such materials may be made up of alumina, titania, silica, zirconia, magnesia, and combinations thereof. In addition, it is stated that the final catalyst, after steam deactivation, has such a pore size distribution that at least 0.4 ml/g of the pore volume is in pores of greater than 90 .ANG. in diameter.
An alternative proposal is put forward in European Patent Application No. 0 176 150. This document recommends the use of a physical admixture of zeolite-containing cracking catalyst particles and alumina particles, stating that the metal contaminants are preferentially captured by the alumina particles and, in consequence, hardly if at all by the zeolite-containing particles. GB Patent No. 2,116,062 likewise recommends the use of alumina particles separately from the zeolite-containing cracking catalyst; these alumina particles have a specific surface area of from 30 to 1000 m.sup.2 /g and a pore volume of from 0.05 to 2.5 ml/g.